There are a variety of microwave filters using dielectric material, and one type of filter includes a resonator using a microstrip line. For example, in the case of a 1/4 wavelength resonator, a rectilinear microstrip line type in-resonator conductor Is provided in a dielectric block, and one end of the in-resonator conductor is opened and the other end is short-circuited to an external grounding conductor provided on an external surface of the dielectric block. To fabricate a filter, a plurality of in-resonator conductors may be arranged in the dielectric block so as to obtain a coupling strength corresponding to the filter characteristics.
Since such in-resonator conductors are disposed in a plane, this structure has limitations in that the area required to mount the filter is difficult to reduce. As an attempt capable of solving this problem, a folded microstrip line type dielectric resonator and dielectric filter have been proposed (for example, Japanese Patent Laid-Open Nos. 5-152,815/1993 and 5-175,702/1993). The dielectric resonator and filter have a structure in which a U-shaped external grounding conductor is provided on the external peripheral face of a dielectric board which excludes both primary surfaces and each conductor is short-circuited at one end only. In this structure, the in-resonator conductor is folded back in a direction perpendicular to its pattern face.
In such structure, since the 1/4 wavelength in-resonator conductor is three-dimensionally folded back so that its entire length is made half or less, it is possible to reduce the area required to mount the dielectric resonator or filter, whereby the size of devices can be reduced. In recent years, further reductions in the sizes of wireless communication units have been demanded, so that a demand for a reduction In the size of the dielectric resonator has become stronger and stronger. However, only the above-described method may be insufficient.
One method for reducing the longitudinal size of the dielectric resonator is to dispose an external grounding conductor in close proximity to the open end of an in-resonator conductor and make use of the phenomenon in which a resonance frequency lowers owing to a load capacity occurring at that place. However, this structure causes an increase in current density at the front end of the in-resonator conductor, thereby causing an increase in loss. As another resonator short-circuit means, a step may be provided on the open-end side of the resonator. This structure, however, is disadvantageous in terms of manufacture and mounting.